The present invention relates to a spark plug of an internal combustion engine. Hereinafter, the term “front” refers to a spark gap side with respect to the direction of an axis of the spark plug, and the term “rear” refers to a side opposite the front side.
A spark plug is generally provided with a cylindrical metal shell, a center electrode and an insulator and mounted on an internal combustion engine for reliable ignition of air-fuel mixture. The insulator has an axial through hole to hold therein the center electrode and includes a front leg portion, a middle body portion, a rear body portion and a large-diameter portion located between and protruded radially outwardly from the rear and middle body portions. The large-diameter portion of the insulator is often formed at a rear end thereof with a rear shoulder section in a curved surface form etc.
The spark plug is exposed to various external stresses such as vibrations due to engine operations and high combustion pressure due to air-fuel mixture combustion. In particular, the rear shoulder section of the insulator is intensively exposed to external stresses. It is thus desired that the insulator has a sufficient level of strength to be protected from cracking under external stresses.
On the other hand, there has been a recent demand for size/diameter reduction of the spark plug. In order to satisfy the spark plug miniaturization demand, it is conceivable to decrease the thickness of the insulator. The insulator however decreases in strength with thickness. As a measure for insulator strength improvement, Japanese Laid-Open Patent Publication No. 2003-7424 proposes to form a glaze layer on a surface of the insulator from the rear body portion to the rear shoulder section so as to reinforce the insulator surface under compressive stress and smoothen fine cracks and holes in the insulator surface. If the glaze layer is too small in thickness, the strength of the insulator cannot be improved to a sufficient level so that there arises a possibility of a crack notably in the rear shoulder section of the insulator. If the glaze layer is too large in thickness, it is likely that a crack will occur on the glaze layer and develop in the insulator. It is thus important to control the thickness of the glaze layer appropriately.